Subsurface igniter



Nov. 20, 1956 H. A. BARCLAY ET AL 2,771,140

SUBSURFACE IGNITER Filed Aug. 28, 1953- 5 Sheets-Sheet 2 FIG. 8

/0- HARRY A. BARCLAY DEAN P. NICHOLS INVENTORS 'A TTORNEY 'bon material.

' ment of our invention.

United States Patent SUBSURFACE IGNITER Harry A. Barclay, Dallas, Tex.,and Dean P. Nichols, Comanche, Okla., assignors, by mesne assignments,to Socony Mobil Oil Company, Inc., a corporation of New York ApplicationAugust 28, 1953, Serial No. 377,155

9 Claims. (Cl. 166-60) This invention relates to an igniter and relatesmore particularly to an electrical igniter for initiating combustion ofhydrocarbon materials in a subterranean formation.

It has been recently proposed to recover hydrocarbon materials fromsubterranean formations by a process involving combustion of a portionof the hydrocarbon material in place within the subterranean formation.this process, oxidizing gas is supplied to the subterranean formationundergoing combustion through an input well leading thereto, and thecombustion gases, the hydrocarbons, and distillation and viscositybreaking products of the hydrocarbons migrate through the formation toan output well or output wells leading from the formation, from whichthese fluids are removed and thereafter treated for recovery of thedesired valuable constituents. Initiation of combustion within theformation is effected by application of heat to the formation along theface thereof at the input well.

It is an object of this invention to provide an igniter for initiatingcombustion of hydrocarbon material within a subterranean formation. Itis another object of this invention to provide an electrical igniterwhich will initiate combustion along the face of an input wellpenetrating a subterranean formation containing hydrocar- It is anotherobject of this invention to provide an electrical igniter for use in awell leading to a subterranean formation which may be suspended in thewell by means of a cable. These and other objects'of the invention willbecome apparent from the following detailed description thereof.

In the drawings Figure 1 is a plan view of an embodi- Figure 2 is alongitudinal sectional view of the upper portion of the igniter ofFigure 1. Figure 3 is a cross-sectional view taken along line 3-3 ofFigure 2. Figure 4 is a cross-sectional view taken along line 44 ofFigure 2. Figure 5 is a crosssectional view taken along line 5-5 ofFigure 2. Figure 6 is a cross-sectional view taken along line 66 ofFigure 2. Figure 7 is a longitudinal, broken plan view of theintermediate portion of the igniter of Figure 1, :showing the casing ofthe heating element in section. Figure 8 is a simplified diagram of theelectrical circuits of the heating elements and the temperature sensingmeans. Figure 9 is a cross-sectional view taken along the line 9-9 ofFigure 7. Figure 10 is a cross-sectional view taken along line 10-10 ofFigure 7. Figure 11 is a longitudinal plan and sectional view of thelower portion of the igniter of Figure 1. Figure 12 is a sectional viewtaken along line 12- 12 of Figure 11. Figure 13 is .a cross-sectionalview taken along line 1313 of Figure 11.

In Figure 1, an igniter 1 embodying the features .of

our invention is shown supported at the lower end of a.

cable 2.

The upper portion of igniter 1 is shown-in detail in Figures 2-6. Cable2, which serves both to support igniter 1 in a well bore and to carrypower leads and leads to temperature sensing means within igniter 1,comprises outer armor 3, inner armor 4, power leads 5, 11 and 12, andleads 13, 14 and 15 to temperature sensing means. Lifting head 20 isprovided with bore 21 which is of sufifb cient diameter that cable 2will readily pass through it.

Adapter 22, which is threaded at 23 into the lower end of head 20, isprovided with screw 24 threaded into adapter 22. Ring 34 may be made ofa rubber or any other satisfactory sealing material which will withstandtemperatures of about 200 F.

Sleeve 35 is screw threaded into the upper end of adapter 31 and extendsupwardly into adapter 22, the external surface of sleeve 35 fitting inclose spaced relation with the inner surface of adapter 22. Screw holes41, three in number at 120 intervals, are provided through the wall ofthe upper end of sleeve 35. Plug assembly 42 is held in position insleeve 35 and adapter 22 by means of set screws 43 which screw throughholes 41 in sleeve 35 into screw holes 44 provided in plug assembly 42.Plug assembly 42 comprises sleeve 45 and body 51 which is pressed intosleeve 45 substantially as shown. Body 51 in the preferred embodiment ofthe invention is formed of a hard, electrical and heat insulatingmaterial such as a phenol condensation product sold under the tradenames of Bakelite or Micarta. The lower end of body 51 is provided withhole 52 which contains screw threads at its lower end and with holes 53,which in this embodiment are six in number spaced at 60 intervals. Screw54 is threadedly engaged in hole 52. Near its upper end sleeve 45 isprovided with external annular channel 55 adapted to receive 0 ring 56which functions to form a seal between sleeve 45 and adapter 22. O ring56 is formed of a rubber or similar material which is capable ofwithstanding temperatures of about 200 F.

Studs 61 are screw threaded into the upper end of sleeve 45 permittingthe securing of body 62 against the upper end of sleeve 45 by means ofset screws 63 which aflix body 62 to studs 61 as shown more clearly inFigure 4. The upper end of body 62 is held in contact with the upper endof the inside of lifting head 21. Outer armor 3 of cable 2 is strippedaway from the cable for a short distance permitting armor 3 to be heldfirmly within the body 62 by cone 64 which fits around cable 2 and hasexternal annular teeth 65 to contact armor 3. Cone 64 is heat treated toprovide a very hard surface to grip armor 3, which in this embodiment isformed of steel, and must, along with inner armor 4, support the weightof the igniter. Cone 64 is held in position by nut 66 which has internalaxial bore 71 and is screw threaded into body 62 at 72. Actual contactof nut 66 against cone 64 is made by washer 73 pressed into the end ofnut 66. Washer 73 is formed of a metal such as brass. Inner armor 4 ofcable 2 is stripped back from the core of cable 2 and positioned withinbore 71. Collar 74,

which fits around cable 2 between inner armor 4 and the core of thecable, prevents the ends of armor 4 contacting and damaging the core ofcable 2. Collar 74 in this embodiment of the invention is formed of aphenol condensation product such as is sold on the market under thetrade name of Bakelite.

Heater support 75 having external annular shoulder 81 'is positionedwithin head adapter 31 with shoulder 81 resting on shoulder 32.Positioned within the upper end of support 75 is insulator 82 andpositioned within the lower. end of support 75 is insulator 83.Insulators 82 and 83 are formed of a material having good dielectricqualities, high compressive strength, and relatively high shearstrength. Insulators 82 and 83 are secured to support 75 by means ofbolts 84. Support 75 and insulators 82 and 83 are provided with holes 85adapted to receive rods 86, 87, and 88 and with holes 92 adapted toreceive leads 131 and 132. Holes 85 through support 75 are enough largerthan rods 86, 87, and 88 to prevent electrical shorts through support 75between the rods. Rods 86, 87, and 88, which are of substantial length,are screw threaded at both ends and are secured at the upper endsthereof to support 75 and insulators 82 and 83 by means of nuts 93 andwashers 94. Washers 94 are countersunk into insulators 82 and 83 to keeprods 86, 87, and 88 centered as they pass through holes 85 in support 75to provide even clearance around the rods so that no electrical shortingwill occur between therods through support 75. Rods 86, 87, and'88 serveto support heating means in the igniter and to conduct electrical powerto said heating means. The rods are formed of stainless steel in thispreferred embodiment of the invention but may be formed of any othersuitable electrically conductive material which will maintain itsstrength and shape and. will have a high resistance to oxidation attemperatures within the range of 1.400, F.

Terminals 95, having screw threads on their lower ends and a shoulder ontheir upper ends, are positioned in holes 53 of body 51 and leads 5, 11,and 12 and 13, 14, and 15 are secured by soldering or other satisfactorymeans to the upper ends of terminals 95. For purposes of clarity, inFigure 2 only leads 11, 12, 13, and 15 are shown secured to four ofterminals 95. Leads and 14 are similarly secured to the remaining twoterminals 95. Seals 101 are positioned around terminals 95 in the lowerend of body 51 as shown. formed of a rubber-like material such as issold on the market under the trade name of Neoprene. Secured on thoseterminals 95 which are connected to leads 5, 11, and 12 are brackets 102which are held in place by means of nuts 103. Conductors 104 areconnected at one end thereof to brackets 102 and at the other end tobrackets 105 by means of bolts 111. Brackets 105 are held on rods 86,87, and 88 by nuts 112.. Thus, by the means just described, power leads5, 11, and 12 are connected to rods 86, 87, and 88.

As previously described, leads 13, 14 and 15 are secured to the upperends of three of terminals 95. Secured to the lower ends of these sameterminals 95 in a conventional manner are leads 113, 114 and 115 whichare connected respectively to terminals 121, 122 and 123 positionedwithin the chamber 124. Connected between terminals 121 and 122 isthermistor element 125. Connected to terminal 121 is thermocouple lead131 and connected to terminal 123 is thermocouple lead 132. Thetemperature sensing leads and their connections have been shown inFigure 2 in diagrammatic form for purposes of clarity. Leads 131 and 132pass into insulating tubing 133 and run through one of holes 92 downwardinto the lower portion of the igniter to thermocouple means hereinafterdescribed. Tube 133 is formed of electrical insulating and heatresistant material and has small separate bores to accommodatethermocouple leads 131 and 132. In the preferred embodiment of theinvention, lead 131 is formed of chromel and lead 132 is formed ofalumel, but it will be apparent to those skilled in the art that leads131 and 132 may be formed of any dissimilar metals satisfactory forthermocouple use and capable of withstanding temperatures in the rangeof 1400 F. Though in this embodiment of the invention only one of holes92 is used to pass leads 131 and 132 to the lower portion of theigniter, it will be apparent that lead 131 may go downward through oneof holes 92 and lead 132 may go downward through another of holes 92 orin this embodiment the presently unused holes 92 may be used foradditional thermocouple leads if desired.

The previously described parts: which are positioned Seals 101 are'block 152 has given satisfactory performance.

within chamber 124 and those within plug assembly 42 are coated duringassembly of the igniter with an electrically insulating grease toprevent moisture condensation upon them. The interior of plug assembly42 is filled with the grease by injecting it through hole 52. Screw 54is screwed into hole 52 to retain the grease within the plug assembly.

Casing 134 is secured by welding or other suitable means to the lowerend of adaptor 31. Casing 134 is of relatively thin-walled constructionin order that it will have low heat capacity and conductance. The lowheat capacity of casing 134 reduces the amount of heat required to bringthe igniter up to the desired operating temperature and permits rapiddissipation of heat to the adjacent oil formation. The low heatconductance feature of casing 134 lessens the amount of heat conductedup the casing to the upper end of the igniter. In this embodiment of theinvention, casing 134 is formed of stainless steel, but it may be madeof any other suitable material which will permit thin-walledconstruction that will retain its strength and shape at the highoperating temperatures used.

Referring to Figure 11, cap 135, provided with central bore 136, issecured by welding or other suitable means to the lower end of casing134. Secured to cap 135 by means of bolts 137 is flange 141. Gasket 142is positioned between cap 135 and flange 141. Flange 141 is providedwith central screw threaded bore 143 having therein seat 144 and radialscrew threaded bore 145 providing communication between bore 143 and theexterior of flange 141. Screw 151 in bore 143 will, when fully screwedinto bore 143, contact seat 144 and close off communication through bore145 into bore 143. Screw 151 may readily be backed ofl untilcommunication can be had from the exterior of flange 141 through bores145 and 143 into the inside of casing 134. Thus, the combination ofbores 143 and 145 and screw 151 comprises a valve through whichcommunication may be had into the interior of casing 134.

In Figures 7 and 11, the disposition of rods 86, 87, and 88 with respectto each other has been shown out of scale for purposes of clarity.Figures 2, 6, 9, 10, 12, and 13 show the true relation of the rods toeach other.

Referring to Figure 7, positioned on rods 86, 87, and 88 downward fromtheir upper ends is heat block 152 which in this embodiment of theinvention is in the form of a lamination having alternate layers of aheat insulating and electrically non-conductive material such asasbestos and -a heat resistant, electrically non-conductive materialsuch as is sold on the market under the trade name Transite. Heat block152 may also be formed of fiber glass wrapped around a ceramic orrefractory core or of any other suitable heat resistant,

electrically non-conductive material. Heat block 152 functions toinsulate the upper end of the igniter from the heating element means andshould, therefore, form a fairly close fit with the inside of casing134, though in practice a clearance of as much as one-sixteenth inchbetween the'inside of casing 134 and the outside of heat The heat blockis held in place on rods 86, 87, and 88 by means of cylindrical shapedspacers 153 and collars 154 which fit on the' rods as indicated. Spacers153 are formed of a ceramic or refractory electrically non-conductivematerial and fit loosely on the rods. In this embodiment collars 154 aremade of stainless steel and secured to the rods by means of set screws.As collars 154 are electrically conductive, it is imperative that theybe secured on the rods in staggered positions, as shown in the drawings,to prevent any possible electrical shorting between the rods which mightoccur during operation.

Core 155 supported on'rods 86, 87, and 88 provides an electricallyinsulating structure which in turn supports electrical heating means.Power supplied to such heating means is transmitted through the igniterby way of the electric circuits, the structural features of which areshown in detail in Figures 2, 7, and 11. A simplified circuit diagramhas been shown in Figure 8 and includes three electrical heaters 160,161, and 162. The power supplied to the heaters 160-162 is provided by athreephase alternating current source 163 to the heating circuit whichis connected in the form of a delta.

The structural features of the heating elements 160 162 and theirmountings are shown in detail in Figures 7 and 912.

Also shown in Figure 8 is a simplified circuit diagram of thetemperature sensing means, the structural features of which are shown indetail in Figures 212 and which include thermocouple element 164,thermistor element 125, voltmeter 170, and ohmmeter 171. Dissimilarconductors 131 and 132 of thermocouple element 164 extend upward intothe upper portion of the igniter, as shown in Figure 2, where theyconnect to leads 13 and 15 which extend to the surface through cable 2.The point of termination of leads 131 and 132 within the upper portionof the igniter forms a cold junction for thermocouple element 164.Connected to and immediately adjacent to the upper end of lead 131 isthermistor element 125 which is also connected to lead 13 which extendsto the surface through cable 2. Voltmeter 170 at the surface isconnected to leads 13 and 15. Ohmmeter 171 at the surface is connectedto leads 13 and 14. Lead 15 is provided with switch 172 which, whenclosed, permits a reading to be made on voltmeter 170 indicative of thetemperature of element 164. Lead 14 is provided with switch 173 which,when closed, permits a reading to be made on ohmmeter 171 indicative ofthe temperature of element 125. Since the cold junction for thermocoupleelement 164 is located adjacent to element 125, the temperature asindicated by element 164 is relative to the temperature of its coldjunction, or, in other words, relative to the temperature of element125. The cold junction temperature is determined by thermistor element125. Thus, the true temperature in the immediate vicinity of element 164is determined by adding the temperature of element 125 to thetemperature of element 164.

Referring to Figure 7, core 155 as previously stated is positioned onrods 86, 87, and 88. Core 155 is formed of a refractory type,electrically non-conductive material, is provided with a continuousspiral groove 174 to receive electrical resistance heating coil 160, andcontains holes 175 for rods 86, 87, and 88, and holes 176 for thetemperature sensing leads. Coil 160 is secured at its upper end to rod86 by means of clamp 181 and is secured at its lower end to rod 88 byclamp 182. Core 155 is spaced downward from heat block 152 by spacers153 'and clamps 154.

Core 183 having continuous spiral groove 184 is positioned on rods 86,87, and 88 spaced downwardly from core 155, and contains holes 185 forrods 86, 87, and 88, and holes 186 for the temperature sensing leads.Spacers 153 and collars 154 are positioned on rods 86 and 87 betweencores 155 and 183. Electrical resistance heating coil 161 is positionedin groove 184 of core 183 and is connected at its upper end to rod 88 byclamp 187 and at its lower end to rod 87 by clamp 191, shown in Figure11.

As shown in Figure 11, core 192 is positioned on rods 86, 87, and 88spaced downwardly from core 183 by means of spacers 153 and clamps 154positioned on rods 86 and 88. Core 192 has continuous spiral groove 193adapted to receive electrical resistance heating coil 162 which isconnected at its upper end to rod 87 by clamp 194 and is connected atits lower end to rod 86 by clamp 195. Passing through core 192 axiallyare holes 196 for rods 86, 87, and 88, and holes 197 for the temperaturesensing leads.

Cores 183 and 192 are identical in material, shape, and dimensions tocore 155.

- Spacers 153 are positionedon rods 87 and 88 below.

core 192. Support 201 is secured on rods 86, 87, and 88 by means of nuts202, which are screw threaded on the lower ends of the rods. Cap 203 issecured to support 201. Support 201 and cap 203 are formed of a heatresistant, electrically non-conductive material.

Shown in Figure 11 positioned within core 192 is thermocouple element164 which is formed by fusing together the ends of conductors 131 and132 just below the point of termination of tubing 133. Thoughthermocouple element 164 is shown here within core 192 which places itwithin the field of influence of heating coil 162, it will be recognizedthat element 164 may be placed anywhere within the heating section ofthe igniter.

The heating coils may be formed of any suitable electrical resistancematerial capable of operating over long periods of time at temperatureswhich will maintain the heating portion of the igniter around 1400 F. Ithas been found with the currently available electrical resistancematerials that the life of the heating coils may be materially extendedby use of coils of a size requiring a relatively low watt density toproduce the operating temperatures of the igniter.

While only three electrical resistance heating coils have been shownhere, additional heating coils may be added in multiples of three tolengthen the igniter as desired. The use of three-phase power requiresthat the number of heaters be in multiples of three to keep the systembalanced electrically. With the addition of heaters, casing 134 and rods86, 87, and 88 will, of course, have to be lengthened proportionately.The length of the rods must be such that when fully expanded atoperating temperature cap 203 will not contact cap and cause the rods tobuckle.

In the operation of our igniter, formations will frequently beencountered wherein the pressures are of such a magnitude that the thinwalls of casing 134 will tend to collapse inwardly. In those instances,the igniter may be pressured with an inert gas such as nitrogen to avalue which will prevent any dangerous stresses being exerted uponcasing 134. This is done by injecting enough gas into the igniter at thesurface that the pressure will increase with the operating temperaturesof the igniter to a value sufiicient to counteract the pressure of theformation in which the igniter is operating. The gas used to pressure upthe igniter is injected in the lower end. of the igniter through bore145, shown in Figure ll. During the initial stages of pressuring theignite-r, the air present Within it may be bled out through hole 25,shown in Figure 2, which is closed by a screw 24 when all air has beenremoved. The pressuring of the igniter with such a gas also functions toplace the heating coils and their related parts in an atmosphere whichaids in reducing their tendency to oxidize.

In operation, the igniter, supported by cable 2, is lowered into a borehole until it is adjacent to a hydrocarbonbearing formation penetratedby the bore hole. Heating coils 168162 are then energized from currentsource 163 to heat the igniter to the temperature necessary to initiatecombustion in the hydrocarbon-bearing formation. The combustionsupporting gas being supplied to the formation through the input welland the hydrocarbon material within the formation being at its ignitiontemperature, combustion of the hydrocarbon material within the formationwill be effected. When combustion within the formation becomesself-sustaining merely with supply of combustion gas thereto, the supplyof electrical current to the heater coils may be discontinued.

In supplying energizing current to the heater coils, it is usuallydesirable to supply current thereto at a slow rate so that thetemperature of the various elements of the heating section of theigniter do not have a high, differential in temperature which would tendto cause damage by buckling or other conditions arising from thermalstrain. For example, satisfactory results have been obtained withoutexcessive thermal strain to the apparatus where the rise in temperaturewas not per-- mitted to exceed 60 F. per hour.

The supply of current to the heating coils to attain the desiredtemperature is controlled by Varying the energy output from source 163and the rise in the temperature is indicated by the reading of meters170 and 171. Similarly, after the igniter has reached operatingtemperature, abrupt changes in temperature either by discontinuance ofthe supply of electrical current to the heating coils or removal of theheater from its position adjacent to the heated walls of the bore holeshould be avoided to prevent damage from thermal strain.

While the apparatus of the invention has been described as being anigniter for initiating combustion of hydrocarbon materials in asubterranean formation, it will be apparent that the apparatus hasgeneral utility with respect to applying heat to such a formation alongthe walls of a bore hole. Thus, for example, the apparatus may beemployed for supplying heat to a subterranean hydrocarbon-containingformation along the walls of a bore hole penetrating the formation toremove parafiin therefrom. In procedures of this sort, of course, it maynot be necessary to supply power to the apparatus to the same extent asit would be supplied where initiation of combustion of a hydrocarbonmaterial is desired. These and other uses of the apparatus of theinvention will be apparent to those skilled in the art.

Having thus described our invention, it will be understood that suchdescription has been given by way of illustration and example and not byway of limitation, reference for the latter purpose being had to theappended claims.

We claim:

1. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises a plurality ofspiral heating elements oriented in an end-to-end array to form anelongated heating unit, supporting structure for said plurality ofelements including three metallic rods electrically insulated one fromthe other and spaced uniformly around the periphcry of a circle withinand smaller than the dimension of said spiral heating elements, andelectrical connections between the two extremities of each of saidheating elements and selected pairs of said rods.

2. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises a head portion, acasing portion secured at its upper end to the lower end of said headportion, closure means secured to the lower end of said casing portion,a cable secured to the upper end of said head portion for supportingsaid apparatus within said bore hole, a plurality of spiral heatingelements positioned within said casing portion and oriented in anend-to-end array to form an elongated heating unit, supporting structurefor said plurality of elements including three metallic rodselectrically insulated one from the other and spaced uniformly aroundthe periphery of a circle within and smaller than the dimension of saidspiral heating elements, electrical connections between the twoextremities of each of said heating elements and selected pairs of saidrods, means for securing said supporting structure pendent within saidcasing portion, a source of three-phase power, means for connecting saidsource of three-phase power through said cable and said head portion tosaid three rods, at least one temperature sensitive element positionedwithin said casing portion for determining the temperature of saidheating elements relative to the temperature of said head portion, atemperature sensitive element within said head portion for determiningthe temperature of said head portion, and conductor means through saidcable to said temperature sensitive elements.

3. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises a head portion; athin walled casing portion secured at its upper end to the lower end ofsaid head portion; closure means secured to the lower end of said casingportion; valve means in said closure means for injecting gas into saidapparatus; port means in the wall of said head portion to permit air tobleed from said apparatus while said gas is being injected into saidvalve means; cable means secured to the upper end of said head portionfor supporting said apparatus in said bore hole; a plurality of corespositioned within said casing portion and oriented in an end-to-endarray spaced apart one from the other, the uppermost of said cores beingspaced downwardly from the upper end of said casing portion, each ofsaid cores having a continuous spiral groove cut into its outer surfaceto receive an electrical resistance heating element and being formed ofheat resistant and electrically insulating material; supportingstructure for said plurality of cores including three metallic rodselectrically insulated one from the other and spaced uniformly aroundthe periphery of a circle within and smaller than the diameter of saidcores; a heat block supported on said rods intermediate said uppermostcore and the upper end of saidcasing portion, said heat block beingformed of a heat and electrically insulating material; means forsupporting said rods pendent within said casing; a plurality ofelectrical resistance heating elements positioned in said spiral groovesof said cores; electrical connections between the two extremities ofeach of said heating elements and selected pairs of said rods; a sourceof three-phase power; conductor means through said cable and said headportion connecting said source of three-phase power to said rods; atleast one temperature sensitive element positioned in said casing withinthe field of influence of said heating elements to determine thetemperature of said heating elements with respect to said head portion;a temperature sensitive element positioned within said head portion todetermine the temperature of said head portion; and conductor meansthrough said cable and said head portion connected to said temperaturesensitive elements.

4. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises three metallicrods positioned parallel to each other and electrically insulated onefrom the other, a plurality of spirally-grooved cores positioned on saidrods in an end to end array spaced apart one from the other, said coresbeing formed of electrically non-conductive and heat resistant materialand having a root diameter larger than the diameter of a circleencompassing said rods perpendicular to the axis of said rods, spacermembers positioned on each of said rods between said cores to maintainsaid cores in position on said rods, electrical resistance heatingelements positioned within said spiral grooves on each of said cores,said heating elements comprising coiled resistance wire, and electricalconnections between the two extremities of each of said heating elementsand selected pairs of said rods.

5. In an apparatus for supplying heat to a subterranean formation, thecombination which comprises a tubular shaped housing closed at bothends, cable means secured to the upper end of said housing forsupporting said housing within a bore hole, three metallic rodspositioned within said housing parallel to the vertical axis of saidhousing and secured pendant from the upper end of said housing, aplurality of spirally grooved cores positioned on said rods in anend-to-end array spaced apart one from the other, said cores beingformed of electrically non-conductive and heat resistant material andhaving a root diameter larger than the diameter of a circle encompassingsaid rods perpendicular to the axis of said rods, spacer memberspositioned on each of said rods between said cores to maintain saidcores in position on said rods, electrical resistance heating elementspositioned within said spiral grooves on each of said cores, saidheating elements comprising coiled resistance wire, electricalconnections between the two extremities of each of said heating elementsand selected pairs of said rods, conductor means within said cable meansconnected to the upper ends of said rods, temperature sensing meanspositioned within said housing within the field of influence of saidheating elements, and conductor means within said cable means connectedto said temperature sensing means.

6. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises a head portion, acasing portion secured to the lower end of said head portion, closuremeans secured to the lower end of said casing portion, a cable securedto the upper end of said head portion for supporting said apparatuswithin a bore hole, three metallic rods positioned within said casingportion parallel to the vertical axis of said casing portion and securedpendant from said head portion, said rods being electrically insulatedone from the other, a plurality of spirally grooved cores positioned onsaid rods in an end-to-end array spaced apart one from the other, saidcores being formed of electrically non-conductive and heat resistantmaterial and having a root diameter larger than the diameter of a circleencompassing said rods perpendicular to the axis of said rods, spacermembers positioned on each of said rods between said cores to maintainsaid cores in position on said rods, electrical resistance heatingelements positioned within said spiral grooves on each of said cores,said heating elements comprising coiled resistance wire, electricalconnections between the two extremities of each of said heating elementsand selected pairs of said rods, electrical conductor means through saidcable connected to the upper ends of said rods, at least one temperaturesensitive element within said casing portion for determining thetemperature of said heating elements relative to the temperature of saidhead portion, a temperature sensitive element within said head portionfor determining the temperature of said head portion, and conductorsthrough said cable connected to said temperature sensitive elements.

7. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises a head portion, athin-walled casing portion secured at its upper end to the lower end ofsaid head portion, closure means secured to the lower end of said casingportion, valve means in said closure means for injecting gas into saidapparatus, port means in the wall of said head portion to permit air tobleed from said apparatus while said gas is being injected into saidvalve means, cable means secured to the upper end of said head portionfor supporting said apparatus in said bore hole, three metallic rodssecured within said casing por tion pendant from said head portion, saidrods being positioned parallel to the vertical axis of said casingportion and electrically insulated one from the other, a plurality ofspirally grooved cores positioned on said rods in an end-to-end arrayspaced apart one from the other, the uppermost of said cores beingspaced downwardly from the upper end of said casing portion, said coresbeing formed of electrically non-conductive and heat resistant materialand having a root diameter larger than the diameter of a circleencompassing said rods perpendicular to the axis of said rods, spacermembers positioned on each of said rods between said cores to maintainsaid cores in position on said rods, an electrical resistance heatingelement positioned within said spiral grooves on each of said cores,said heating element comprising coiled resistance wire, electricalconnections between the two extremities of said heating element andselected pairs of said rods, a heat block supported on said rodsintermediate the uppermost of said cores and the upper end of saidcasing portion, said heat block being formed of a heat and electricallyinsulating material,

conductor means through said cable and said head portion for connectingthe upper ends of said rods to a source of three-phase power, at leastone thermocouple positioned in said casing within the field of influenceof said heating element to determine the temperature of said heatingelement with respect to said head portion, the cold junction of saidthermocouple being positioned within said head portion, a thermistorpositioned within said head portion adjacent the cold junction of saidthermocouple to determine the temperature of said head portion and thecold junction of said thermocouple, and conductor means through saidcable in said head portion connected to said thermistor and the coldjunction of said thermocouple.

8. In an apparatus for supplying heat to a subterranean formationadjacent a bore hole, the combination which comprises three metallicrods positioned parallel to each other and electrically insulated onefrom the other, a plurality of spirally grooved cores positioned on saidrods in an end-to-end array, said cores being formed of electricallynon-conductive and heat resistant material and having a root diameterlarger than the diameter of a circle encompassing said endsperpendicular to the axis of said rods, electrical resistance heatingelements positioned within said spiral grooves on said cores, saidheating elements comprising coiled resistance wire, electricalconnections between the two extremities of each of said heating elementsand selected pairs of said rods, and spacer members positioned on eachof said rods between cores to maintain apart and in position on said rodcores containing within spiral grooves thereof each of said heatingelements.

9. In an apparatus for supplying heat to a subterranean formation, thecombination which comprises a tubular shaped housing closed at bothends, cable means secured to the upper end of said housing forsupporting said housing within a bore hole, three metallic rodspositioned within said housing parallel to the vertical axis of saidhousing and secured pendant from the upper end of said housing, aplurality of spirally grooved cores positioned on said rods in anend-to-end array, said cores being formed of electrically non-conductiveand heat resistant material and having a root diameter larger than thediameter of a circle encompassing said rods perpendicular to the axis ofsaid rods, electrical resistance heating elements positioned within saidspiral grooves on said cores, said heating elements comprising coiledresistance wire, electrical connections between the two extremities ofeach of said heating elements and selected pairs of said rods, spacermembers positioned on each of said rods to maintain apart and inposition on said rod cores containing Within said spiral grooves thereofeach of said heating elements, conductor means within said cable meansconnected to the upper ends of said rods, temperature sensing meanspositioned within said housing within the field of influence of saidheating elements, and conductor means within said cable means connectedto said temperature sensing means.

References Cited in the file of this patent UNITED STATES PATENTS957,785 Linquest May 10, 1910 1,726,041 Powell Aug. 27, 1929 2,506,853Berg et a1 May 9, 1950 2,632,836 Ackley Mar. 24, 1953 2,647,196Carpenter et a1 July 28, 1953

